The uneven heating and cooling of glass is the main cause of stress spots. Only in the design and operation of tempering furnaces, find out the factors that affect the uneven heating and cooling plus various factors, strengthen process control, and strictly observe operating procedures. Reduce the occurrence of stress spots.
1. Heating factor
The glass is unevenly heated in the plane direction before being quenched. After the glass with uneven heating is quenched and cooled, the compressive stress will be small in the high temperature area, and the compressive stress will be large in the low temperature area. The unevenly distributed compressive stress will be generated on the glass surface.
The main factors that lead to uneven heating are the difference in heater power. The area with high heating power has high temperature. The area with low heating power has low temperature. The heater fails to heat normally. The temperature sensor fails or is calibrated. The temperature setting is not reasonable. The side of the furnace body has more heat dissipation, the temperature setting should be slightly higher than the middle, the glass absorbs more heat in the middle than the side, and the temperature at the middle side is slightly higher.
These factors are that if not reflected in the setting, it will lead to uneven heating; the glass is unreasonably placed, and because the roller table is a poor conductor of heat, such as repeated use of an area will cause the temperature of the area to decrease without glass being placed, The temperature in this area is relatively high. The temperature of the next batch of glass in this area is relatively high; the heating furnace is not well sealed, and there is more local heat dissipation, which causes uneven temperature.
2. Cooling factor
The tempering process of glass is rapid cooling after heating. The cooling process is as important as the heating process to the formation of tempering stress. The uneven cooling of the glass in the plane direction before the quenching is the same as the uneven heating. It also causes uneven stress. The surface compressive stress formed in a region with a large cooling intensity is large, and the compressive stress formed in a region with a small cooling intensity is small. Uneven cooling will cause unevenly distributed stress on the glass surface.
The main factors that lead to uneven cooling are the unreasonable distribution of the air nozzles of the fans and the uneven gap of the air nozzles, which makes the air pressure distribution on the glass surface uneven, the cooling of the glass surface is uneven, the air nozzles are blocked, and some of the air nozzles cannot supply air normally. The heat conduction capacity of the channel is too large, and the part contacting the roller table is rapidly cooled, and the first cooled part will form a large compressive stress.
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